The present invention relates to a funnel constituting a glass bulb for a cathode ray tube.
The glass bulb for a cathode ray tube comprises a front panel on which an image is displayed, and a rear funnel which is sealed to the panel for forming a glass envelope. As shown in FIG. 6, the funnel 1 comprises an open end portion 10 in an approximately rectangular shape similar to the panel, a neck portion 11 in a cylindrical shape for mounting an electron gun, a yoke portion 12 on which a deflection coil is externally mounted, and a body portion 13 in a funnel-shape which gradually changes in a direction from the open end portion 10 to the yoke portion 12. The joint portion between the yoke portion 12 and the body portion 13 is generally called TOR (Top Of Round).
The contour of a cross section P(z) of the body portion 13 in parallel with the open end portion 10 is a rectangle similar to the open end portion 10 in the vicinity of the open end portion 10 (z=0), and is generally a circle in the vicinity of the yoke portion 12 (z=T). The contours of the inner surface and the outer surface of the cross section P(z) respectively comprise three arcs, which are arcs RL constituting longer sides, arcs RS constituting shorter sides, and arcs RD in contact with the longer side and the shorter side.
An angle d (°) defined between a diagonal axis (DA) and the major axis (LA) is 36.87° when the aspect ratio of the display surface is 4:3, and the angle d is 29.36° when the aspect ratio is 16:9. The center of the arc RD is conventionally set on the diagonal axis (DA) for the sake of design, so that the outermost part D(z) of the contour of the cross section P(z) exists on the diagonal axis (DA) as well. In such funnel 1, since the contour of the body portion 13 suddenly changes in the vicinity of the diagonal axes (DA), ridge-like corner shapes approximately parallel with the diagonal axis are formed especially on the longer side of the diagonal axes. Especially, because the wide screen type funnel having the aspect ratio of 16:9 has the larger ratio of the longer side to the shorter side, the ridge-like corner shapes are remarkable.
Generally a press molding is used for manufacturing the funnel. As shown in FIG. 7, after a certain amount of molten glass gob is supplied into a bottom mold 20, a plunger (not shown) is pressed against the molten glass gob, thereby the molten glass gob is extended in a gap between the bottom mold 20 and the plunger to mold the funnel. FIG. 7 shows the funnel 1 in a state of completion of pressing and extending (in a state of fill up). FIG. 8 shows the funnel 1 in a state of midway of pressing and extending the molten glass gob in the bottom mold 20. Arrows 14 in FIG. 8 shows a direction of pressing and extending the molten glass. The molten glass is pressed and extended up to the open end portion on a minor axis (SA), the major axis (LA), and the diagonal axis (DA) in this order.
As described before, the ridge-like corner shapes approximately parallel with the diagonal axis are formed in the vicinity of the diagonal axes of the funnel, and these shapes inhibit the pressing and extending of the glass when the funnel is press-molded. Namely, as shown by the arrow 14 in FIG. 8, the glass is extended in the direction of the diagonal axis while wrapping around both from the minor axis side and the major axis side. However, since a resistance against the pressing and extending increases at the ridge-like corner shapes, the fill-up of the glass up to the open end portion is delayed at the corner shapes in comparison with the rest portions.
Since the fill-up is delayed on the diagonal axes in this way, the temperature of the glass filled in the open end portion in the vicinity of the diagonal axes decreases, so that the problems such as generation of small cracks in the glass, increase of time required for the fill-up, or increase of pressing pressure occur. The ridge-like corner shapes are also disadvantageous in terms of strength. Namely, scratches generated as a result of handling the funnel tend to concentrate at the ridge-like corner shapes. Also, when the funnel is used to constitute a cathode ray tube, the inside of the funnel is evacuated, so that a vacuum stress generated from a difference between an inner air pressure and an outer air pressure tends to concentrate at the ridge-like corner shapes. It is undeniable that the cathode ray tube may break depending on degree of the scratches and the vacuum stress.